1. Field of the Invention
This invention relates to a new class of ester or ester liquid crystal compounds. More particularly, it relates to esters and sulfur esters having a biphenyl moiety and which exhibit liquid crystal properties.
2. Description of the Prior Art
Various liquid crystal compositions have been disclosed in the prior art. A typical desired specification for a field effect nematic liquid crystal display composition includes a temperature range of from about 0.degree. to about 60.degree. C., with the lower temperature indicating the melting point of the material and the upper temperature indicating the clearing or nematic isotropic temperature. Second, a threshold voltage of from about 1.0 to about 1.5 volts and an operating voltage of from about 1.5 volts to about 2.5 volts is desired. Third, the composition should exhibit sufficient optical contrast for display purposes when incorporated in a liquid crystal display structure. Fourth, the lifetime of the liquid crystal composition should be at least about five years. Fifth, the resistivity of the composition should be at least about 5.times.10.sup.9 ohm centimeters.
Various ester liquid crystal compounds are known in the art, such as the phenylbenzoate esters described in U.S. Pat. No. 3,876,286. While such compounds are known to be highly stable, they generally have higher melting points and narrower nematic temperature ranges than other types of liquid crystal compounds, such as Schiff bases. Broadening of the operating temperature range for ester liquid crystal compounds may be achieved by preparing a blend of different esters, each contributing a characteristic feature to the overall composition. In order to meet the needs of the rapidly developing, highly competitive liquid crystal technology, there remains a requirement for new compounds that may be used to tailor liquid crystal compositions with desired properties, particularly for increased clearing points and reduced operating voltages.